The long-term aim of this proposal is to understand the molecular basis of the modification of enzyme activities and the regulation of enzyme biosynthesis as exemplified by the proteins involved in the biosynthesis of the aromatic amino acids, tyrosine, phenylalanine, and tryptophan in Escherichia coli K12. DAHP (3-deoxy-D-arabinoheptulosonic acid 7-phosphate)-synthase (EC 4.1.2.15) is the first enzyme of the pathway which leads to the synthesis of these three amino acids. There are three DAHP synthase isoenzymes in E. coli, one of which is feedback-inhibited by tyrosine, one by phenylalanine, and one by tryptophan. We obtained two of the isoenzymes in pure form. When we determined their molecular properties, unexpectedly large differences were found between their subunit molecular weights and their quaternary structures. This could indicate an example of convergent evolution. In order to test this hypothesis, my first objective is to initiate primary structural analysis of the two isoenzymes. The respective structural genes for the three isoenzymes, aroF, aroG, and aroH, are scattered over the E. coli chromosome and are not subject to a common regulatry mechanism. While regulation of aroG expression seems to be solely by repression, we have shown that aroF expression is more complex. My second objective is to elucidate differences in the regulatory mechanisms for the expression of aroF and aroG by DNA sequence analysis of their respective regulatory regions. Finally, there is unpublished evidence that one of the isoenzymes is modified in response to different growth conditions, that are unrelated to feedback inhibition. To elucidate the nature of this modification is another objective.